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Publication# Approche de conception et d’optimisation de centrale solaire intégrée à cycle combiné inspirée de la méthode du pincement (partie II: réseau d’échangeurs de chaleur)

Résumé

Steam production units (HRSG and HSSG) of ISCCS include several heat exchangers (economizers, evaporator, superheater, reheaters, etc.). The knowledge of the extended composites as a function of the solar input, allows the determination of the most critical zones for heat transfer but does not allow, in itself, the full knowledge of the real streams needed to be able to design an optimum heat exchanger network. The procedure proposed in this paper permits, from so called interaction factors which characterize the interdependancy between streams, the determination of the massflows in each stream. The choice of the best heat exchanger network must respect, for each operational conditions, the optimum evaporation levels (including pressures et temperatures) determined in part I, as well as the particular practical operational factors (independance or not between the various heat recovery units, etc.). The network design is done using the standard guidelines of pinch technology (respect of the minimum pinch DTmin for each heat exchanger close to the pinch temperature, separate design of the zone above and below the pinch temperature, etc.). The respect of the DTmin in the critical zones of heat transfer requires the use of stream splitting and the network includes heat exchanger tubes which are interlaced at the same level of the stack. One example of the best performing power plant designed on the basis of this approach is given.

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Chaudière de récupération

Une chaudière de récupération est un échangeur de chaleur qui permet de récupérer l’énergie thermique d’un écoulement de gaz chaud. Elle produit de la vapeur qui peut être utilisée dans un procédé ind

Échangeur de chaleur

Un échangeur de chaleur est un dispositif permettant de transférer de l'énergie thermique d'un fluide vers un autre sans les mélanger. Le flux thermique y traverse la surface d'échange qui sépare les

Transfert thermique

vignette|alt=Autour d'un feu, des mains reçoivent sa chaleur par rayonnement (sur le côté), par convection (au-dessus de ses flammes) et par conduction (à travers un ustensile en métal).|Les modes de

Daniel Favrat, John Richard Thome, Olivier Zürcher

A summary of the objectives and conclusions reached in this project are: •A litterature search on post dryout heat transfer for pure refrigerants and for refrigerant-oil mixtures was performed and a comprehensive review written (Chapters 9, 10, and 11); •The correct thermodynamic definition of the heat transfer coefficient for evaporation of refrigerant-oil mixtures was presented and then used in the project, i.e. defining the heat transfer coefficient using the local bubble point temperature Tbub of the refrigerant-oil mixture rather than with the saturation temperature of the pure refrigerant Tsat incorrectly used in prior studies (Chapter 3); •A new generalized approach, that is simple to implement, was developed for predicting bubble point temperatures of refrigerant-oil mixtures for miscible oils that can be applied to any refrigerant-oil combination and is accurate over the local oil concentration range from 0-50 wt.% oil range confronted in direct- expansion evaporators with 0-5 wt.% oil in their refrigerant charge (Chapter 4); •A new thermodynamic method for preparation of temperature-enthalpy- vapor quality (T-h-x) curves for refrigerant-oil mixtures for general application was developed and validated against test data (Chapters 5 and 6), including a simple expression for calculating local oil concentrations as a function of vapor quality; •New, accurate thermodynamic methods for preparation of temperature-enthalpy-vapor (T-h-x) curves for the refrigerant blend R-407C and R-407C/oil mixtures were developed for thermal design of evaporators and for the present experimental test, and was validated against measured test data (Chapter 12); •Recommendations were made on how to include oil effects and refrigerant-oil T-h-x curves into evaporator design methods (Chapter 7); •The effects of oil on set-point parameters for control devices were analyzed and some recommendations made (Chapter 8); •An online oil concentration measurement system was perfected using a high accuracy density flowmeter that allowed oil concentrations to be measured accurately and continuously during experiments, such that oil holdup in the heat transfer test sections at high vapor quality could be identified and the correct inlet oil concentrations be measured, and thus utilized to reduce experimental data; a simpler industrial method for applying the measurement system was also developed and presented (Chapter 15); •Experimental results (local heat transfer coefficients, two-phase pressure drops and some two-phase flow patterns) were obtained for R-134a and R-134a/oil mixtures evaporating in plain and microfinned tubes over a wide range of test conditions (Chapters 16 and 17); •Experimental results (local heat transfer coefficients, two-phase pressure drops and some two-phase flow patterns ) were obtained for R-407C and R- 407C/oil mixtures evaporating in plain and microfinned tubes over a wide range of test conditions (Chapters 18 and 19); •The recently proposed flow boiling model and flow pattern map of Kattan-Thome-Favrat, that importantly models heat transfer coefficients based on local flow pattern and predicts the onset of dryout in horizontal tubes and local heat transfer coefficients in partial dryout regimes, was described in detail; the flow pattern map is also useful to designers wishing to design in specific flow pattern regimes and avoid inefficient heat transfer regimes, such as mist flow and stratified flow (Chapters 20 and 21); •The stratified-wavy flow regime model was modified based on the new test data at high vapor quality for pure R-134a and R-407C, which increased the accuracy of the method significantly in this flow regime, now referred to as the “modified” Kattan- Thome-Favrat flow boiling model (Chapter 22); •The modified Kattan- Thome-Favrat flow boiling model and flow pattern map were shown to accurately predict R-407C heat transfer and flow pattern data and hence is recommended for industrial use for designing direct-expansion evaporators with refrigerant blends [the original version was also verified for R-402 and R-404A blends, see Chapter 21] (Chapter 22); •The effect of local refrigerant-oil viscosity on liquid-phase convection in flow boiling heat transfer and on flow patterns was successfully incorporated into the modified Kattan-Thome-Favrat model, establishing the first flow boiling design method that predicts oil effects on heat transfer (with quite reasonable accuracy too), and included a simple method for calculating the values of µref-oil for use in the model (Chapter 23); •The oil effects on microfin heat transfer coefficients were analyzed and heat transfer augmentation ratios were determined for R-134a/oil and R-407C/oil mixtures (Chapter 25); •Plain tube, two-phase pressure drop gradients for R-134a and R-407C at three mass velocities were compared to the Friedel frictional two-phase pressure drop correlation, attaining accurate results and also microfin pressure drop augmentation ratios were determined (Chapter 25); •The ratio of [µoil/µref]mw, derived from the Arrhenius logrithmic viscosity mixing rule and local oil concentration w, was shown to accurately predict the effect of oil on two-phase pressure drop gradients at high vapor qualities, with an empirical exponent m = 0.18355 found for R-134a/oil mixtures (no foaming) and an empirical expression for m for R-407C/oil mixtures (with foaming). This ratio incorporated into the Friedel correlation accurately predicts two-phase pressure drops of evaporating refrigerant-oil flows (Chapter 25).

1996The pinch analysis method is a useful tool for the energy integration of industrial processes (structuring and simplification using simple guidelines). The pinch analysis method, extended to include exergy factors, falls within the framework of a global multidisciplinary analysis which considers in light of a sustainable development, economic, energetic, exergetic, and environmental factors. One of the essential themes of such a global framework is the system’s life cycle analysis. This analysis considers the manufacture, the exploitation and the recycling of components (from cradle to grave). Such considerations as well as pressure drops in heat exchangers were not included in the original pinch method, which centered primarly on economic and heat transfer aspects. The extension of the pinch analysis method to include exergy factors related to some of the considerations mentioned above, leads to a global exergy balance which includes irreversibility considerations due to heat transfer, dissipation and the manufacture of components. The thermodynamic optimisation of heat exchangers based on an optimal distibution of exergy losses is realized, and the grey exergy assiociated with the manufacture of shell and tube heat exchangers is calculated as an example. Further extension of the pinch method to include an electrical energy balance was also realized. Such a balance is particularly useful when intoducing heat pumps or power units. The extended composite curves which result from the extension above offer a graphic representation of all the exergy losses of the process using a Carnot factor versus heat rate diagram and an electric power versus Carnot factor diagram. With such diagrams, the choice of the optimal pinch value (Tmin) will be determined for the minimum total exergy loss of the process. The heat exchanger network design is also based on an exergetic criterion (the difference of global Carnot factor) which allows an exergetic optimisation of the position of the heat exchanger. A procedure for selecting the different heat exchanger alternatives is included in the proposed design method. This procedure limits the number of network designs having the best chances to arrive at the optimal network (network with the minimum global exergy losses). The extended pinch analysis method proposed here has been applied to typical industrial processes with acceptable, consistent and sometimes differents results from those obtained with the original pinch method. This enrichment of the method will lead to better designs.

1995In this master thesis report the development of an innovative spiral heat exchanger based on polymer materials is described. Building prototypes, erection of a test bench and firsts tests of the heat exchanger are presented. The heat exchanger prototype survived all tests especially several days in contact with aggressive gases. A facility integrating a Diesel exhaust gases production has been developed to test this heat exchanger design. Performance results obtained during the tests are analysed. Measurement acquisition software developed with Labview was also used. Challenges have been overcome to run the facility in stable conditions in order to obtain reliable measurement. Heat load recovery achieved with the presented heat exchanger is in the range of 1.5 kW thermic but potential heat recovery about 3.5kW might be achievable. Overall heat transfer coefficient is improved compared to other polymer based heat exchanger design. Pressure drop on gas channel is in the range of several mbar and must be further improved and fouling must be minimized. Such a design based on polymer film technology provides better corrosion and chemical resistance compared to conventional metal heat exchangers. Due to the smooth surface of polymer film fouling is reduced. Series production and usage of such heat exchangers would allow operating low temperature waste heat recovery in gases at affordable costs. One promising application is heat recovery in soiled gases in combination with ORC power generation.

2012